Saturday, February 8, 2014

atDNA: What Do Cousin Matches Look Like

The RootsTech 2014 presentation on DNA was very interesting, and I plan on watching it again when the video becomes available. I do think one point that should have been brought up wasn't. The presenter made it sound like all atDNA matches are related within the last 5 generations. This is also the genetic distance which the testing companies use as the cut off point when suggesting relationships. They don't suggest relationships past 5 generations. If you read the full explanation of possible relationships twenty generations is a possibility when looking at distant matches.
I decided to take a look at our atDNA matches that I've confirmed a shared ancestors with to see the patterns of inheritance for each generation.
Most of these charts compare my mother and myself to various confirmed surname matches.
We'll start by comparing myself with my first cousin:

First Cousin Match

The Blue lines represent the DNA we match them on for each chromosome 

I don't have any second cousin matches

Third Cousin Matches

These 3rd cousin matches vary by quite a bit. I share a great deal of DNA in common with my 3rd cousin, but my mother shares less DNA with her 3rd cousin matches. It could be her matches are 3rd cousin once removed?

Fourth Cousin matches

On the left are my mother's fourth cousin matches, on the right are mine. My mother's large block share (the blue) may be a 4th cousin, while the orange match is definitely 4th cousin once removed.

Fifth Cousin Matches

Here we see that a fifth cousin can still share good size blocks of DNA. This first is a comparison between my fifth cousin and I; the second is a comparison between my cousin, and our shared fifth cousin.

Seventh Cousin matches

Here we see at the 7th cousin range we can still inherit blocks of atDNA. My mother has matches here with single IBD blocks ranging 7.93 to 16.15 cm. 

Tenth Cousin Match

This 10th cousin is one of my matches on my father's side. She still shares quite a bit of DNA with me at this distance. She has a long pedigree chart I've compared with and I can't find any other surnames that we have in common.
Add caption
The conclusion I draw from this is that generally if you share large amounts of DNA with someone you are closely related. First cousins are very obvious matches. I don't have second cousin matches so I can't say how large the shares would be? By third cousin we see a greater variations in the amount of DNA shared. It's definitely difficult to differentiate cousin relationships after first cousin. It's still possible to share sizable blocks of DNA up to 10 generations. I have many matches who have large posted trees at Family Tree DNA and we can't find common ancestors leading me to believe that a majority of my matches are beyond the 5 generation cut off. 

No comments: